High rigidity, high damping capcity cast iron

ABSTRACT

A high rigidity, high damping capacity cast iron, which is a cast iron containing 3 to 7% of Al, and produced by heating at 280 to 630° C. after casting, and then cooling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No. 12/940,140 filed Nov. 5, 2010. U.S. application Ser. No. 12/940,140, filed Nov. 5, 2010 is a Continuation Application of International Application No. PCT/JP2009/058705, filed May 8, 2009. International Application No. PCT/JP2009/058705, filed May 8, 2009 is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-142932, filed May 30, 2008. The entirety of all of the above-listed Applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high rigidity, high damping capacity cast iron having a high Young's modulus and high vibration damping capacity. The cast iron of the present invention is used as a structural material for machine tools and high-precision machine tools required to have rigidity or for precise measurement instruments in which Young's modulus and vibration are matters of concern. Such use makes it possible to increase the machining efficiency of the material, and the accuracy and precision of the work.

2. Description of the Related Art

Heretofore, flaky graphite cast iron, which is relatively high in vibration damping capacity, has been mainly used as a structural material for machine tools. Flaky graphite cast iron contains a large amount of flaky graphite and therefore has a complex-type vibration damping mechanism. Therefore, it has higher damping capacity than steel or the like and has advantageous characteristics in terms of formability and cost for manufacture of large structural materials. Incidentally, researches have been made on other materials having a high damping capacity, such as concrete-based materials, natural granite, and CFRP, for use as structural materials for machine tools in place of flaky graphite cast iron. However, none of these materials have been actually used due to low rigidity, a problem with formability or cost, or the like.

Now, flaky graphite cast iron is widely used as a structural material for machine tool beds, tables, columns, and the like, since it is advantageous in terms of damping properties, castability and cost. However, machine tools for machining hard-to-work materials, which are severely work-hardened, are required to have high rigidity such that large cutting can be stably maintained and to have a high damping capacity such that harmful vibration can be prevented. Therefore, in some cases where much higher vibration-damping capacity is desired, conventional flaky graphite cast iron cannot achieve sufficient machining efficiency or sufficient work accuracy due to the influence of vibration.

Conventional flaky graphite cast iron used for machine tools and the like, such as FC 300, contains a large amount of flaky graphite, which produces a complex-type damping mechanism. Thus, it is a structural material superior in vibration damping capacity among conventional materials. The vibration damping capacity of such flaky graphite cast iron can be improved by increasing the amount of flaky graphite. However, there is a problem that as flaky graphite cast iron increases, the dynamic Young's modulus (hereinafter simply referred to as Young's modulus) decreases. The graphite content of flaky graphite cast iron can be controlled by controlling the amounts of C and Si. If a structural material for machine tools has a low Young's modulus, the structural material must be made thick so that a certain level of rigidity can be maintained. This is not preferred, because of not only a problem with structural design but also an increase in cost.

To improve the vibration damping capacity, methods have been proposed in which bainite or martensite is formed in the base structure of flaky graphite cast iron (Casting Engineering 68 (1996) 876). In these methods, however, as the vibration damping capacity increases, Young's modulus decreases, and it is difficult to improve both of them at the same time. For example, Patent Documents 1, 2, and 3 disclose methods for improving the vibration damping capacity. Patent Documents 1 to 3 all disclose a method for improving the logarithmic decrement.

Patent Documents 1 to 3 show the results of measurement of vibration damping capacity. However, nothing about Young's modulus is described in these documents, and the value of Young's modulus is unknown from the documents. Specifically, Patent Documents 1 and 2 relate to brake materials, and therefore, it is considered that in these documents, Young's modulus is not indispensable, but the strength is rather important. Particularly, Patent Document 1 discloses that an object of its invention is to provide a brake material having strength as high as that of gray cast iron and having damping capacity equal to or higher than that of gray cast iron. Patent Document 3 discloses that aluminum-containing, damping cast iron for improving damping performance has been invented in view of an improvement in the damping performance of machine tools or precision machining equipment. However, although for the purpose of maintaining the machine accuracy, it is indispensable to maintain the rigidity of the structural material, the document discloses nothing about it.

It is known from Patent Documents 1 to 3 that the vibration damping capacity can be improved by the addition of aluminum. However, when reviewed in detail, the disclosed methods are different. Specifically, Patent Document 1 produces a brake material high in a vibration damping capacity and strength by heat-treating cast iron added with aluminum, at an A₁ transformation temperature or higher (910 to 1,000° C.) and adjusting the cooling rate so as to form pearlite at an area ratio of 70% or more. In Patent Document 2, the vibration damping capacity is improved by the effect of adding A₁, and by making a hypereutectic composition to increase the amount of graphite and to form micro-pores. However, this method is considered to significantly reduce Young's modulus. Patent Document 3 is an example in which the vibration damping capacity is improved by the addition of aluminum. However, this document is silent about Young's modulus. The methods disclosed in Patent Documents 1 to 3 do not necessarily improve Young's modulus and the vibration damping capacity at the same time, and therefore, the vibration damping capacity must be further improved.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 63-140064

Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 2001-200330

Patent Document 3: Jpn. Pat. Appln. KOKAI Publication No. 2002-348634

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a high rigidity, high damping capacity cast iron. The cast iron consists of a cast iron containing 3 to 7% of Al. And the cast iron is produced by heating at 280 to 630° C. after casting, and then cooling.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a characteristic chart illustrating the relationship between heat treatment temperature and damping performance improvement ratio.

FIG. 2 is a characteristic chart showing the relationship between Young's modulus and the logarithmic decrement of Al-containing flaky graphite cast iron.

FIG. 3 is a characteristic chart showing the relationship between Young's modulus and the logarithmic decrement of Al and Sn-containing flaky graphite cast iron.

DETAILED DESCRIPTION OF THE INVENTION

An object of the present invention is to provide high-rigidity, high-damping-capacity cast iron whose vibration damping capacity is further improved with compatibility between Young's modulus and vibration damping capacity, which has been an issue in the prior art, and which thus has a high level of Young's modulus and vibration damping capacity. Specifically, an object of the present invention is to provide high-rigidity, high-damping-capacity cast iron that has the same level of Young's modulus as that of conventional flaky graphite cast iron, which is high in vibration damping capacity, and also has significantly high vibration damping capacity.

A high rigidity, high damping capacity cast iron according to the present invention (first invention) is a cast iron containing 3 to 7% of Al and produced by heating at 280 to 630° C. after casting, and then cooling. More specifically, the first invention is a cast iron comprising 3 to 7% of Al, 0.25 to 1.0% of Mn, 0.04% or less of P, 0.03% or less of S, and the balance of Fe and inevitable impurities, and being produced by heating at 280 to 630° C. after casting, and then cooling.

A high rigidity, high damping capacity cast iron according to the present invention (second invention) is a cast iron containing 3 to 7% of Al and 0.03 to 0.20% of Sn and produced by heating at 280 to 630° C. after casting, and then cooling. More specifically, the second invention is a cast iron comprising 3 to 7% of Al, 0.25 to 1.0% of Mn, 0.04% or less of P, 0.03% or less of S, 0.03 to 0.20% of Sn, and the balance of Fe and inevitable impurities, and being produced by heating at 280 to 630° C. after casting, and then cooling.

Further, a high rigidity, high damping capacity cast iron according to the present invention (third invention) is a cast iron containing 3 to 7% of Al, and C and Si in such amounts that a carbon equivalent represented by formula (1) below is from 3.30 to 3.95, and being produced by heating at 280 to 630° C. after casting, and then cooling.

Carbon equivalent (%)=C content (%)+(⅓)×Si content (%)  (1)

More specifically, the third invention is a cast iron consisting of C and Si in such amounts that the carbon equivalent represented by formula (1) above is from 3:30 to 3.95, 3 to 7% of Al, 0.25 to 1.0% of Mn, 0.04% or less of P, 0.03% of less of S, 0.03 to 0.20% of Sn, and the balance of Fe and inevitable impurities, and being produced by heating at 280 to 630° C. after casting, and cooling.

According to the present invention, there is provided a high-rigidity, high-damping-capacity cast iron whose vibration damping capacity is further improved with compatibility between Young's modulus and vibration damping capacity, and which thus has a high level of Young's modulus and vibration damping capacity. More specifically, there is provided a high-rigidity, high-damping-capacity cast iron that has the same level of Young's modulus as that of conventional flaky graphite cast iron, which is high in vibration damping capacity, and also has significantly high vibration damping capacity.

The present invention is further described in more detail below.

To solve the problems associated with Patent Documents 1 to 3, the inventors of the present invention have previously proposed a high rigidity, high damping capacity cast iron in which the relationship between the carbon equivalent, and the C content and the Si content is set (Jpn. Pat. Appln. KOKAI Publication No. 2008-223135). However, it has been found that the sufficient damping capacity cannot be achieved thereby.

Under the circumstances, the inventors have made further improvements to accomplish the present invention.

As the Al (aluminum) content increases, the vibration damping capacity of flaky graphite cast iron (a high rigidity, high damping capacity cast iron) improves but reaches a limit. For example, when the vibration damping capacity and Young's modulus are measured as the Al content is gradually increased, improvements of them are observed from an Al content of 3%, but the vibration damping capacity rather becomes lower as the Al content becomes higher than 7%. However, the inventors have found that the addition of tin (Sn) to the Al-containing flaky graphite cast iron improves Young's modulus and the vibration damping capacity. In addition, the inventors have revealed that the vibration damping capacity and Young's modulus can be significantly changed by controlling the carbon equivalent (C.E.), the C/Si weight ratio, the Al content, and the Sn content of flaky graphite cast iron. In order to improve the vibration damping capacity with the level of Young's modulus maintained, the C.E., the C/Si weight ratio, the Al content, and the Sn content described in claims should be adequately controlled.

In the present invention, the Al content is defined to be from 3 to 7% for the reasons described below. From 3%, the Al content starts to have an advantageous effect on the vibration damping capacity of Al and Sn-containing flaky graphite cast iron. If the Al content is less than 3%, almost no improvement effect can be observed. If it becomes 6% or more, the vibration damping capacity may gradually decrease, and if it exceeds 7%, the vibration damping capacity may further decrease. If the Al content is more than 7%, iron aluminum carbide formed by the addition of Al becomes hard and brittle, so that the material becomes fragile and less workable. For the reasons described above, the adequate Al content is set at 3 to 7%.

The mechanism of the improvement in the vibration damping capacity of flaky graphite cast iron by the addition of Al can be explained by the theory that the improvement is due to the formation of an iron alloy in which Al is solid-solutioned (former) or by the theory that the improvement is due to the formation of an iron aluminum carbide (latter). The inventors' research takes the latter theory. These theories both assume that the substance formed can produce a ferromagnetic damping mechanism.

In the present invention, the Sn content is defined to be from 0.03 to 0.2% for the reasons described below. If the Sn content is too low, the effect of improving Young's modulus and the vibration damping capacity cannot be observed. From about 0.03%, the content becomes effective in improving Young's modulus and the vibration damping capacity, and when the content is around 0.08%, the effect becomes most significant. As the Sn content increases, the effect gradually decreases, and when the content becomes 0.2% or more, the effect significantly decreases, so that the improvement effect cannot be obtained. Thus, the adequate Sn content is from 0.03 to 0.2%. Sn is an important additive element, because it can improve not only Young's modulus and the vibration damping capacity but also the tensile strength.

While there may be various theories on the mechanisms of the improvement effect by the addition of Sn, the present inventors consider as follows. It is said that when Al is added to flaky graphite cast iron, iron aluminum carbide is formed by reaction of iron and Al with carbon. It is also said that the iron aluminum carbide, which is a ferromagnetic substance, produces a ferromagnetic type vibration damping mechanism. According to the inventors' study, when the addition amount of Al is increased, the amount of the iron aluminum carbide increases, but does not increase when the Al content reaches about 6%. However, the amount of the iron aluminum carbide formed is always larger in the case where Sn is also added than in the case where only Al is added. It is considered that the improvement effect by the addition of Sn is brought about accordingly.

In the present invention, the high rigidity, high damping capacity cast iron of the invention contains elements other than Al and Sn, such as C, Si, Mn, P, and S. The C content and the Si content are described in detail later.

The Mn content should be from 0.25 to 1.0% as in the case of conventional flaky graphite cast iron. The Mn content should be in the above range, because if the Mn content is 0.25% or more, the cast iron can have increased strength and hardness, but if the Mn content is more than 1.0%, the cast iron may be chilled so that it may be made hard and brittle.

The P content should be 0.04% or less as in the case of conventional flaky graphite cast iron. The P content should be in the above range, because if the P content is more than 0.04%, P may react with iron to form steadite, a hard compound, which makes the cast iron brittle.

The S content should be 0.03% or less as in the case of conventional flaky graphite iron. This is because if the S content is more than 0.03%, the molten metal may have poor fluidity, and the cast iron may be chilled so that it may be made hard and brittle.

In the third invention, the carbon equivalent represented by formula (1) above is from 3.30 to 3.95%. As the carbon equivalent increases, the vibration damping capacity increases, but Young's modulus decreases. Both of them cannot be simultaneously improved by increasing or decreasing the carbon equivalent, but the carbon equivalent should be set at an adequate value, because it has a significant effect on the vibration damping capacity and Young's modulus. When Al is added, the eutectic composition, at which the eutectic reaction between austenite and graphite occurs, changes from that of conventional flaky graphite cast iron. In conventional flaky graphite cast iron, the eutectic reaction occurs when the carbon equivalent represented by formula 1 is 4.3%. However, when Al is added, the eutectic reaction may occur at a carbon equivalent smaller than this value. When the carbon equivalent is larger than the value of the eutectic composition, the cast iron can be hypereutectic to have a significantly reduced Young's modulus, which is not preferred.

In the present invention, if the carbon equivalent (C.E.) exceeds 3.95%, the vibration damping capacity may be significantly improved, but Young's modulus may be significantly reduced. This may be because at such a carbon equivalent, the cast iron exceeds the eutectic composition to have a hypereutectic composition. On the other hand, if the carbon equivalent is small, the amount of the formation of graphite will be small so that Young's modulus may be improved. In this case, however, the vibration damping capacity may be reduced, and therefore, the carbon equivalent should be 3.3% or more. Thus, the carbon equivalent is defined to be from 3.30 to 3.90.

In the present invention, the temperature of the heat treatment conducted after the casting is from 280 to 630° C. The improvement of the performance by heating and cooling significantly varies with the heating temperature. The effect of the heat treatment is shown in FIG. 1. While FIG. 1 shows cases where Al and Sn were added in the production of the materials according to the present invention, almost the same tendency was observed when only Al was added. When the heat treatment temperature is lower than 280° C. or is more than 630° C., the effect of the heat treatment is small.

Thus, it is preferred that the heat treatment be conducted at a temperature in the range where the ratio of improvement of the damping capacity is 5% or more, i.e., in the range of 280 to 630° C., before cooling. The temperature range where the effect is improved by 20% or more is from 360 to 580° C. A high effect can be produced in these temperature ranges, and the highest effect can be obtained by heating to 500° C. followed by cooling. The cooling method may be any of furnace cooling and air cooling. The reason why the damping capacity is improved by the heat treatment is not clear.

The heat treatment process may vary with the process performed after the casting of the product according to the present invention. For example, when the cast product with as-cast surfaces is used as it is, the heat treatment should be performed after the casting. For example, when the cast product is machined into a product with a predetermined size before use, the heat treatment is most preferably performed after the machining. However, if the heat treatment cannot be performed after machining for a certain reason, the heat treatment may be performed before the machining.

Specific Examples of the present invention and Comparative Examples are described below.

EXAMPLES 1 TO 8 AND COMPARATIVE EXAMPLES 1 TO 8

The composition of cast iron was controlled using a high-frequency melting furnace. A cast iron ingot produced with FC 300, a carburizing agent, ferromanganese, and silicon carbide were added to a graphite crucible and melted. Thereafter, the carbon content and the silicon content were adjusted with ferrosilicon and a carburizing agent, and thus about 20 kg of molten metal were obtained. The Al content and the tin content of the obtained cast product were adjusted by the addition of ferroaluminum and pure tin, respectively. The melting temperature was set at about 1,450° C. Before tapping, a Ca—Si—Ba-based inoculant was added to the melt, and then the melt was cast into a furan self-hardening mold of φ30×300 mm.

The resulting cast product was worked into a size of 4×20×200 mm and then measured for logarithmic decrement as an index of vibration damping capacity and for dynamic Young's modulus. In this case, a comparison was made between heat-treated and non-heat-treated products. In Examples 1 to 8, the Al-containing cast iron was heat-treated, while in Comparative Examples 1 to 8, the Al-containing cast iron was not heat-treated. The test method was according to JIS G 0602. Specifically, the test piece was suspended from two points and given 1×10⁻⁴ of strain amplitude by an electromagnetic vibrator. The vibration was then stopped for free damping, and the logarithmic decrement and the dynamic Young's modulus were determined. The characteristics of the resulting cast products are shown in Table 1 below. The logarithmic decrement is a value measured when the strain amplitude of the vibration was 1×10⁻⁴. Although Table 1 shows nothing about P or S, P<0.025, and S<0.020. Different Examples with the same composition mean that they are from the same melt but different cast samples.

TABLE 1 Hardness Dynamic Young's Logarithmic decrement C Si Mn Al Sn C.E. % (HB) modulus (GPa) (×10⁻⁴) Example 1 3.03 1.96 0.79 5.60 — 3.68 253 124 299 Example 2 3.03 1.96 0.79 5.60 — 3.68 256 124 275 Example 3 2.90 2.04 0.80 5.58 — 3.58 263 128 244 Example 4 2.90 2.04 0.80 5.58 — 3.58 259 128 247 Example 5 3.01 1.86 0.76 5.48 — 3.63 266 130 231 Example 6 3.01 1.86 0.76 5.48 — 3.63 266 130 215 Example 7 3.03 1.87 0.77 6.00 — 3.65 271 131 246 Example 8 3.03 1.87 0.77 6.00 — 3.65 268 130 256 Comparative 3.03 1.96 0.79 5.60 — 3.68 253 120 216 Example 1 Comparative 3.03 1.96 0.79 5.60 — 3.68 255 123 219 Example 2 Comparative 2.90 2.04 0.80 5.58 — 3.58 263 125 179 Example 3 Comparative 2.90 2.04 0.80 5.58 — 3.58 263 127 193 Example 4 Comparative 3.01 1.86 0.76 5.48 — 3.63 266 127 172 Example 5 Comparative 3.01 1.86 0.76 5.48 — 3.63 288 128 176 Example 6 Comparative 3.03 1.87 0.77 6.00 — 3.65 271 128 184 Example 7 Comparative 3.03 1.87 0.77 6.00 — 3.65 274 128 199 Example 8 (Composition in units of mass %)

Among the data shown in Table 1, Young's modulus and the logarithmic decrement of each sample are plotted to illustrate the relationship therebetween in FIG. 2. When Young's modulus and the logarithmic decrement are evaluated at the same time, the comparison is easy to understand in FIG. 2. Although the values of Young's modulus and the logarithmic decrement values of the respective samples vary, the average values are indicated as a straight line. In FIG. 2, line a represents the data of Examples 1 to 8, and line b the data of Comparative Examples 1 to 8. When importance is attached to the vibration damping capacity of FC 250 and FC 300, cast irons currently used, they exhibit a Young's modulus of about 120 GPa. For comparison with them, therefore, Young's modulus data in the range of 115 to 130 GPa are shown in the drawing.

FIG. 2 shows that concerning the Young's modulus-logarithmic decrement characteristics, the performance of the heat-treated products according to the present invention is improved by about 40% as compared with Comparative Examples 1 to 8 (non-heat-treated products). This value indicates that the performance of the cast iron according to the present invention is about 2.5 to 3.0 times higher than that of FC 250 or FC 300, cast iron currently used (which shows a logarithmic decrement of about 100×10⁻⁴, when Young's modulus is 120 PGa).

EXAMPLES 9 TO 16 AND COMPARATIVE EXAMPLES 9 TO 16

The melt was cast into a furan self-hardening mold of φ30×300 mm using the same process as in Examples 1 to 8 and Comparative Examples 1 to 8.

The resulting cast product was worked into a size of 4×20×200 mm and then measured for logarithmic decrement as an index of vibration damping capacity and for dynamic Young's modulus. In this case, a comparison was made between heat-treated and non-heat-treated products. In Examples 9 to 16, the Al and Sn-containing cast iron was heat-treated, while in Comparative Examples 9 to 16, the Al and Sn-containing cast iron was not heat-treated. The test method was according to JIS G 0602. Specifically, the test piece was suspended from two points and given 1×10⁻⁴ of strain amplitude by an electromagnetic vibrator. The vibration was then stopped for free damping, and the logarithmic decrement and the dynamic Young's modulus were determined. The characteristics of the resulting cast products are shown in Table 2 below. The logarithmic decrement is a value measured when the strain amplitude of the vibration was 1×10⁻⁴. Although Table 2 shows nothing about P or S, P<0.025, and S<0.020. Different examples with the same composition mean that they are from the same melt but different cast samples.

TABLE 2 Dynamic Logarithmic Hardness Young's decrement C Si Mn Al Sn C.E. % (HB) modulus (GPa) (×10⁻⁴) Example 9 3.05 1.85 0.80 5.98 0.077 3.67 262 123 342 Example 10 3.05 1.85 0.80 5.98 0.077 3.67 260 126 326 Example 11 3.07 2.04 0.77 6.07 0.076 3.75 255 117 404 Example 12 3.07 2.04 0.77 6.07 0.076 3.75 252 117 387 Example 13 3.00 1.88 0.81 6.00 0.072 3.63 262 119 360 Example 14 3.03 1.94 0.76 5.78 0.069 3.68 262 121 404 Example 15 2.96 1.86 0.80 5.97 0.072 3.58 259 126 295 Example 16 2.96 1.86 0.80 5.97 0.072 3.58 260 130 271 Comparative 3.05 1.85 0.80 5.98 0.077 3.67 262 120 295 Example 9 Comparative 3.05 1.85 0.80 5.98 0.077 3.67 265 123 268 Example 10 Comparative 3.07 2.04 0.77 6.07 0.076 3.75 255 114 347 Example 11 Comparative 3.07 2.04 0.77 6.07 0.076 3.75 250 114 328 Example 12 Comparative 3.00 1.88 0.81 6.00 0.072 3.63 262 116 280 Example 13 Comparative 3.03 1.94 0.76 5.78 0.069 3.68 262 118 379 Example 14 Comparative 2.96 1.86 0.80 5.97 0.072 3.58 259 124 278 Example 15 Comparative 2.96 1.86 0.80 5.97 0.072 3.58 262 125 200 Example 16 (Composition in units of mass %)

Among the data shown in Table 2, Young's modulus and the logarithmic decrement of each sample are plotted to illustrate the relationship therebetween in FIG. 3. When Young's modulus and the logarithmic decrement are evaluated at the same time, the comparison is easy to understand in FIG. 3. Although the values of Young's modulus and the logarithmic decrement values of the respective samples vary, the average values are indicated as a straight line. In FIG. 3, line a represents the data of Examples 9 to 16, and line b the data of Comparative Examples 8 to 16. When importance is attached to the vibration damping capacity of FC 250 and FC 300, cast irons currently used, they exhibit a Young's modulus of about 120 GPa. For comparison with them, therefore, Young's modulus data in the range of 115 to 130 GPa are shown in the drawing.

FIG. 3 shows that concerning the Young's modulus-logarithmic decrement characteristics, the performance of the heat-treated products according to the present invention is improved by about 30% as compared with Comparative Examples 9 to 16 (non-heat-treated products). This value indicates that the performance of the cast iron according to the present invention is about 3.5 times higher than that of FC250 or FC300, cast iron currently used (which shows a logarithmic decrement of about 100×10⁻⁴, when Young's modulus is 120 PGa).

The present invention are not limited by the very embodiments described above, and in the practice of the present invention, the composition of Al, Sn, C, Si, Mn, P, S, or the like may be appropriately changed without departing from the gist of the present invention. Further, different compositions described in the embodiments may be appropriately used in combination.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A method of producing a high rigidity, high damping capacity cast iron, comprising: casting a cast iron comprising 3 to 7 mass % of Al; heating the cast iron at a temperature falling within a range of 280 to 630° C.; and cooling the cast iron.
 2. The method according to claim 1, wherein the cast iron comprises 3 to 7 mass % of Al, 0.25 to 1.0 mass % of Mn, 0.04 mass % or less of P, 0.03 mass % or less of S, and the balance of Fe and inevitable impurities.
 3. The method according to claim 1, wherein the temperature falls within a range of 360 to 580° C.
 4. A method of producing a high rigidity, high damping capacity cast iron, comprising: casting a cast iron comprising 3 to 7 mass % of Al and 0.03 to 0.20 mass % of Sn; heating the cast iron at a temperature falling within a range of 280 to 630° C.; and cooling the cast iron.
 5. The method according to claim 4, wherein the cast iron comprises 3 to 7 mass % of Al, 0.25 to 1.0 mass % of Mn, 0.04 mass % or less of P, 0.03 mass % or less of S, 0.03 to 0.20 mass % of Sn, and the balance of Fe and inevitable impurities.
 6. The method according to claim 4, wherein the temperature falls within a range of 360 to 580° C.
 7. A method of producing a high rigidity, high damping capacity cast iron, comprising: casting a cast iron comprising 3 to 7 mass % of Al, and C and Si in such amounts that a carbon equivalent represented by formula (1) below is from 3.30 to 3.95; heating the cast iron at a temperature falling within a range of 280 to 630° C.; and cooling the cast iron, wherein formula (1) is as follows: carbon equivalent (mass %)=C content (mass %)+(⅓)×Si content (mass %)
 8. The method according to claim 7, wherein the cast iron comprises C and Si in such amounts that the carbon equivalent represented by the formula (1) is from 3.30 to 3.95, 3 to 7 mass % of Al, 0.25 to 1.0 mass % of Mn, 0.04 mass % or less of P, 0.03 mass % or less of S, 0.03 to 0.20 mass % of Sn, and the balance of Fe and inevitable impurities.
 9. The method according to claim 7, wherein the temperature falls within a range of 360 to 580° C. 